The invention relates to an arrangement to increase the thermal fatigue resistance of glass tubes through which fluid flows and are pressure-loaded and the use of such an arrangement in a heat exchanger or an absorber tube for solar collectors.
Tubular components made from glass are frequently used to guide a flow medium in heat exchangers and absorber tubes of solar collectors. The thermal fatigue resistance presents a substantial problem, whereby said thermal fatigue resistance can be described as a function of the type of glass, the geometrical design and the dimensioning. Such glass tubes must usually exhibit a minimum wall thickness due to the pressure prevailing in the interior. This leads to the fact that extreme temperature changes of the fluid flowing in the interior can lead to glass breaks due to the thermal conducting behavior of the tube wall and the associated stresses. In particular when filling an absorber tube made from glass in high-evacuated solar collectors, which is filled during downtime conditions with a cold heat transfer medium, temperature change of approx. 250 K can occur. This leads then to the fact that the inside of the glass tube cools down significantly, however a high difference between the interior circumference and the outer circumference of the glass tube can be observed due to the thermal conducting behavior in the wall of the glass tube. The ensuing unacceptable high temperature gradient in the wall of the glass tube usually causes a damage of the glass tube due to the associated stresses, which can often lead to a complete break. In order to solve this problem so far, essentially two solution courses were followed. In accordance with a first solution fluid contact with the inner wall of the glass tube was avoided by a hydraulically separated fluid-containing tube, which was slid into the glass tube, whereby said separate tube was connected heat-conclusively to the glass tube by suitable dimensioning and/or additional heat-transferring measures. A substantial disadvantage of this design consisted in the associated higher costs as well as a poor heat transfer between the glass tube and the fluid, as the heat transfer could not take place directly, but took place first at the wall of the fluid-containing tube and then via convection in the gas contained in the gap and then again heat transfer took place from the wall of the glass tube to the inner wall of the glass tube respectively in reverse order with a cooled down flow medium.
In accordance with a second solution it was directed to the use of high-quality glasses, which already exhibit a sufficient thermal fatigue resistance. This solution is however characterized by substantially higher costs, whereby the availability of such materials must be ensured. Furthermore it is not possible to use known standardized prefabricated simple glass tubes for certain standard applications, but only these special, high-quality tubes.